Aryl dicarboxylic acid diimidazole-based compounds as n-type semiconductor materials for thin film transistors

ABSTRACT

A thin film transistor comprises a layer of organic semiconductor material comprising an organic semiconductor material that comprises an aryl dicarboxylic acid diimidazole-based compound. Such transistors can further comprise spaced apart first and second contact means or electrodes in contact with said material. Further disclosed is a process for fabricating ac thin film transistor device, preferably by sublimation or solution-phase deposition onto a substrate, wherein the substrate temperature is no more than 150° C.

FIELD OF THE INVENTION

The present invention relates to the use of aryl dicarboxylic aciddiimidazole-based compounds as semiconductor materials in n-channelsemiconductor films for thin film transistors. The invention relates tothe use of these materials in thin film transistors for electronicdevices and methods of making such transistors and devices.

BACKGROUND OF THE INVENTION

Thin film transistors (TFTs) are widely used as a switching element inelectronics, for example, in active-matrix liquid-crystal displays,smart cards, and a variety of other electronic devices and componentsthereof. The thin film transistor (TFT) is an example of a field effecttransistor (FET). The best-known example of an FET is the MOSFET(Metal-Oxide-Semiconductor-FET), today's conventional switching elementfor high-speed applications. Presently, most thin film devices are madeusing amorphous silicon as the semiconductor. Amorphous silicon is aless expensive alternative to crystalline silicon. This fact isespecially important for reducing the cost of transistors in large-areaapplications. Application of amorphous silicon is limited to low speeddevices, however, since its maximum mobility (0.5-1.0 cm²/V sec) isabout a thousand times smaller than that of crystalline silicon.

Although amorphous silicon is less expensive than highly crystallinesilicon for use in TFTs, amorphous silicon still has its drawbacks. Thedeposition of amorphous silicon, during the manufacture of transistors,requires relatively costly processes, such as plasma enhanced chemicalvapor deposition and high temperatures (about 360° C.) to achieve theelectrical characteristics sufficient for display applications. Suchhigh processing temperatures disallow the use, for deposition, ofsubstrates made of certain plastics that otherwise might be desirablefor flexible displays.

In the past decade, organic materials have received attention as apotential alternative to inorganic materials such as amorphous siliconfor use in semiconductor channels of TFTs. Organic semiconductormaterials are simpler to process, especially those that are soluble inorganic solvents and, therefore, capable of being applied to large areasby far less expensive processes, such as spin-coating, dip-coating, inkjet printing, and microcontact printing. Furthermore, organic materialsmay be deposited at lower temperatures, opening up a wider range ofsubstrate materials, including plastics, for flexible electronicdevices. Accordingly, thin film transistors made of organic materialscan be viewed as a potential key technology for plastic circuitry indisplay drivers, portable computers, pagers, memory elements intransaction cards, and identification tags, where ease of fabrication ormechanical flexibility are important considerations.

Organic materials for use as potential semiconductor channels in TFTsare disclosed, for example, in U.S. Pat. No. 5,347,144 to Garnier etal., entitled “Thin-Layer Field-Effect Transistors with MIS StructureWhose Insulator and Semiconductors Are Made of Organic Materials.”

Organic semiconductor materials that can be used in TFTs to provide theswitching and/or logic elements in electronic components typicallyrequire significant mobilities, well above 0.01 cm²/Vs, and currenton/off ratios (hereinafter referred to as “on/off ratios”) greater than1000. Organic TFTs having such properties are capable of use forelectronic applications such as pixel drivers for displays andidentification tags. However, most of the compounds exhibiting thesedesirable properties are “p-type” or “p-channel,” meaning that negativegate voltages, relative to the source voltage, are applied to inducepositive charges (holes) in the channel region of the device. N-typeorganic semiconductor materials can be used in TFTs as an alternative top-type organic semiconductor materials, where the terminology “n-type”or “n-channel” indicates that positive gate voltages, relative to thesource voltage, are applied to induce negative charges in the channelregion of the device.

Moreover, one important type of TFT circuit, known as a complementarycircuit, requires an n-type semiconductor material in addition to ap-type semiconductor material. See Dodabalapur et al. in “Complementarycircuits with organic transistors” Appl. Phys. Lett. 1996, 69, 4227. Inparticular, the fabrication of complementary circuits requires at leastone p-channel TFT and at least one n-channel TFT. Simple components suchas inverters have been realized using complementary circuitarchitecture. Advantages of complementary circuits, relative to ordinaryTFT circuits, include lower power dissipation, longer lifetime, andbetter tolerance of noise. In such complementary circuits, it is oftendesirable to have the mobility and the on/off ratio of an n-channeldevice to be similar in magnitude to the mobility and the on/off ratioof a p-channel device. Hybrid complementary circuits using an organicp-type semiconductor and an inorganic n-type semiconductor are known, asdescribed in Dodabalapur et al. (Appl. Phys. Lett. 1996, 68, 2264.), butfor ease of fabrication, an organic n-channel semiconductor materialwould be desired in such circuits.

Only a limited number of organic materials have been developed for useas a semiconductor n-channel in TFTs. One such materialbuckminsterfullerene C60 exhibits a mobility of 0.08 cm²/Vs but isconsidered to be unstable in air. See R. C. Haddon, A. S. Perel, R. C.Morris, T. T. M. Palstra, A. F. Hebard and R. M. Fleming, “C₆₀ Thin FilmTransistors” Appl. Phys. Let. 1995, 67, 121. Perfluorinated copperphthalocyanine has a mobility of 0.03 cm²/Vs, and is generally stable toair operation, but substrates must be heated to temperatures above 100°C. in order to maximize the mobility in this material. See “NewAir-Stable n-Channel Organic Thin Film Transistors” Z. Bao, A. J.Lovinger, and J. Brown J. Am. Chem., Soc. 1998, 120, 207. Othern-channel semiconductors, including some based on a naphthaleneframework, have also been reported, but with lower mobilities. SeeLaquindanum et al., “n-Channel Organic Transistor Materials Based onNaphthalene Frameworks,” J. Am. Chem., Soc. 1996, 118, 11331. One suchnaphthalene-based n-channel semiconductor material,tetracyanonaphthoquino-dimethane (TCNNQD), is capable of operation inair, but the material has displayed a low on/off ratio and is alsodifficult to prepare and purify.

Aromatic tetracarboxylic diimides, based on a naphthalene aromaticframework, have also been demonstrated to provide, as an n-typesemiconductor, n-channel mobilities greater than 0.1 cm²/Vs usingtop-contact configured devices where the source and drain electrodes areon top of the semiconductor. Comparable results could be obtained withbottom contact devices, that is, where the source and drain electrodesare underneath the semiconductor, but a thiol underlayer needed to beapplied between the electrodes, which had to be gold, and thesemiconductor. See Katz et al. “Naphthalenetetracarboxylic Diimide-Basedn-Channel Transistor Semiconductors: Structural Variation andThiol-Enhanced Gold Contacts” J. Am. Chem. Soc. 2000 122, 7787; “ASoluble and Air-stable Organic Semiconductor with High ElectronMobility” Nature 2000 404, 478; Katz et al., European Patent ApplicationEP1041653 or U.S. Pat. No. 6,387,727. In the absence of the thiolunderlayer, the mobility was found to be orders of magnitude lower inbottom-contact devices. Relatively higher mobilities have been measuredin films of perylene tetracarboxylic diimides having linear alkyl sidechains using pulse-radiolysis time-resolved microwave conductivitymeasurements. See Struijk et al. “Liquid Crystalline Peryllene Diimides:Architecture and Charge Carrier Mobilities” J. Am. Chem. Soc. Vol 2000,122, 11057. However, initial devices based on materials having aperylene framework used as the organic semiconductor led to devices withlow mobilities, for example 10⁻⁵ cm²/Vs for perylene tetracarboxylicdianhydride (PTCDA) and 1.5×10⁻⁵ cm²/Vs for NN′-diphenyl perylenetetracarboxylic acid diimide (PTCDI-Ph. See Horowitz et al. in “Evidencefor n-Type Conduction in a Perylene Tetracarboxylic Diimide Derivative”Adv. Mater. 1996, 8, 242 and Ostrick, et al. J Appl. Phys. 1997, 81,6804.

US Patent Pub. No. 2002/0164835 A1 to Dimitrakopoulos et al. disclosesimproved n-channel semiconductor films made of perylene tetracarboxylicacid diimide compounds, one example of which isN,N′-di(n-1H,1H-perfluorooctyl) perylene-3,4,9,10-tetracarboxylic aciddiimide. Substituents attached to the imide nitrogens in the diimidestructure comprise alkyl chains, electron deficient alkyl groups,electron deficient benzyl groups, the chains preferably having a lengthof four to eighteen atoms. U.S. Pat. No. 6,387,727 B1 to Katz et al.discloses fused-ring tetracarboxylic diimide compounds, one example ofwhich is N,N′-bis(4-trifulormethyl benzyl)naphthalene-1, 4, 5,8,-tetracarboxylic acid diimide. Such compounds are pigments that areeasier to reduce.

There is a need in the art for new and improved organic semiconductormaterials for transistor materials and improved technology for theirmanufacture and use. There is especially a need for n-type semiconductormaterials exhibiting significant mobilities and current on/off ratios inorganic thin film transistor devices.

SUMMARY OF THE INVENTION

The present invention relates to the use, in n-channel semiconductorfilms for thin film transistors, aryl dicarboxylic diimidazole-basedcompounds as represented by the following structures:

wherein Ar₁ and Ar₂ are the same or different, and are each carbocyclicor heterocyclic aromatic ring systems, each comprising 4 to 60 carbonatoms, fused to the adjacent imidazole ring, wherein each of Ar₁, andAr₂ are substituted with one or more fluorine atoms; X is a substituentorganic or inorganic group at any available position on the corenucleus; n is zero or an integer from 1 to 8 and m is zero or an integerfrom 1 to 4. The Ar₁ and Ar₂ moieties can be single rings or condensedor fused aromatic polycyclic ring systems, including carbocyclic,heterocyclic, or hybrid ring systems in which a carbocylic ring is fusedto a heterocylic ring.

Such films are capable of exhibiting field-effect electron mobilitygreater than 10⁻³ cm²/Vs in the film form. Such semiconductor films arealso capable of providing device on/off ratios in the range of at least10³.

Another aspect of the present invention is the use of such n-channelsemiconductor films in thin film transistors, each such transistorfurther comprising spaced apart first and second contact connected to ann-channel semiconductor film, and a third contact means spaced from saidfirst and second contact means that is adapted for controlling, by meansof a voltage applied to the third contact means, a current between thefirst and second contact means through said film. The first, second, andthird contact means can correspond to a drain, source, and gateelectrode in a field effect transistor. More specifically, an organicthin film transistor (OTFT) has an organic semiconductor layer. Anyknown thin film transistor construction option is possible with theinvention.

Another aspect of the present invention is directed to a process forfabricating a thin film transistor, preferably by sublimation orsolution-phase deposition of the n-channel semiconductor film onto asubstrate.

Furthermore, preferred compounds used in the present invention possesssignificant volatility so that vapor phase deposition, where desired, isavailable to apply the n-channel semiconductor films to a substrate inan organic thin film transistor.

As used herein, “a” or “an” or “the” are used interchangeably with “atleast one,” to mean “one or more” of the element being modified.

As used herein, the terms “over,” “above,” and “under” and the like,with respect to layers in the inkjet media, refer to the order of thelayers over the support, but do not necessarily indicate that the layersare immediately adjacent or that there are no intermediate layers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent when taken in conjunction with thefollowing description and drawings wherein identical reference numeralshave been used, where possible, to designate identical or analogousfeatures that are common to the figures, and wherein:

FIG. 1 illustrates a cross-sectional view of a typical organic thin filmtransistor having a bottom contact configuration;

FIG. 2 illustrates a cross-sectional view of a typical organic thin filmtransistor having a top contact configuration;

DESCRIPTION OF THE INVENTION

Cross-sectional views of typical organic thin film transistors are shownin FIGS. 1 and 2, wherein in FIG. 1 illustrates a typical bottom contactconfiguration and FIG. 2 illustrates a typical top contactconfiguration.

Each thin film transistor (TFT) in FIGS. 1 and 2 contains a sourceelectrode 20, a drain electrode 30, a gate electrode 44, a gatedielectric 56, a substrate 28, and the semiconductor 70 of the inventionin the form of a film connecting the source electrode 20 to drainelectrode 30, which semiconductor comprises a compound selected from theclass of arylimidazole-based compounds described herein.

When the TFT operates in an accumulation mode, the charges injected fromthe source 20 into the semiconductor are mobile and a current flows fromsource to drain, mainly in a thin channel region within about 100Angstroms of the semiconductor-dielectric interface. See A. Dodabalapur,L. Torsi H. E. Katz, Science 1995, 268, 270, hereby incorporated byreference. In the configuration of FIG. 1, the charge need only beinjected laterally from the source 20 to form the channel. In theabsence of a gate field the channel ideally has few charge carriers; asa result there is ideally no source-drain conduction.

The off current is defined as the current flowing between the sourceelectrode 20 and the drain electrode 30 when charge has not beenintentionally injected into the channel by the application of a gatevoltage. For an accumulation-mode TFT, this occurs for a gate-sourcevoltage more negative, assuming an n-channel, than a certain voltageknown as the threshold voltage. See Sze in Semiconductor Devices—Physicsand Technology, John Wiley & Sons (1981), pages 438-443. The on currentis defined as the current flowing between the source 20 and the drain 30when charge carriers have been accumulated intentionally in the channelby application of an appropriate voltage to the gate electrode, and thechannel is conducting. For an n-channel accumulation-mode TFT, thisoccurs at gate-source voltage more positive than the threshold voltage.

It is desirable for this threshold voltage to be zero, or slightlypositive, for n-channel operation. Switching between on and off isaccomplished by the application and removal of an electric field fromthe gate electrode 44 across the gate dielectric 56 to thesemiconductor-dielectric interface, effectively charging a capacitor.

In accordance with the invention, the organic semiconductor materialsused in the present invention, when used in the form of an n-channelfilm, can exhibit high performance under ambient conditions without theneed for special chemical underlayers.

The improved n-channel semiconductor film of the present invention,comprising arylimidazole-based compounds described herein, is capable ofexhibiting a field effect electron mobility greater than 10⁻⁴ cm²/Vs,preferably greater than 10⁻³ cm²/Vs. In addition, the n-channelsemiconductor film of the invention is capable of providing on/offratios of at least 10², advantageously at least 10³. The on/off ratio ismeasured as the maximum/minimum of the drain current as the gate voltageis swept from zero to 80 volts and the drain-source voltage is held at aconstant value of 80 volts, and employing a silicon dioxide gatedielectric.

Moreover, these properties are attainable after repeated exposure of then-type semiconductor material to air, before film deposition, as well asexposure of the transistor device and/or the channel layer to air afterdeposition.

The n-channel semiconductor materials used in the present inventionoffer advantages a class of compounds that have not been assemiconductor in a TFT.

The lowest lying unoccupied molecular orbital of the compound is at anenergy that allows for injection of electrons at useful voltages frommetals with reasonable work functions. This conjugated structuregenerally has a desirable lowest unoccupied molecular orbital (LUMO)energy level of about 3.5 eV to about 4.6 eV with reference to thevacuum energy level. As known in the art, LUMO energy level andreduction potential approximately describe the same characteristics of amaterial. LUMO energy level values are measured with reference to thevacuum energy level, and reduction potential values are measured insolution versus a standard electrode. An advantage for deviceapplications is that the LUMO in the crystalline solid, which is theconduction band of the semiconductor, and the electron affinity of thesolid, are both measured with reference to the vacuum level. The latterparameters are usually different from the former parameters, which areobtained from solution.

In one embodiment of the present invention, the arylimidazole-basedcompounds are represented by the following structures aryl dicarboxylicdiimidazole-based compounds having 6 to 10 fused rings as represented bythe following structures:

wherein Ar₁ and Ar₂ are the same or different, and are each carbocyclicor heterocyclic aromatic ring systems, comprising 4 to 60 carbon atoms,fused to each corresponding adjacent imidazole ring in the structure,wherein each of Ar₁, and Ar₂ are substituted with at least one or morefluorine atoms; X is a substituent organic or inorganic group at anyavailable position on the core nucleus; n is an integer from 1 to 8 andm is an integer from 1 to 4. The Ar₁ and Ar₂ moieties can be singlerings or condensed or fused aromatic polycyclic ring systems, includingcarbocyclic, heterocyclic, or hybrid ring systems in which a carbocylicring is fused to a heterocyclic ring. For example, Ar₁ and Ar₂ can be abenzo, naphthaleno, peryleno, thiopheno, pyridino, furano, pyrazolo, oranthraceno moiety.

In another preferred embodiment, Ar₁ and Ar₂ are substituted orun-substituted benzo, benzothiopheno, quinolino, thiopheno, ornaphthaleno moieties each substituted with one or more fluorine-atoms.Examples of fluorine-containing groups include fluorinated carbocyclicor heterocyclic aromatic rings preferably having 5-10 ring atoms, morepreferably 5 to 6 ring atoms (most preferably phenyl), or anycombinations thereof. Finally, in yet another preferred embodiment, Ar₁and Ar₂ are each a benzo moiety substituted by one or morefluorine-containing groups.

Additional optional substituents on Ar₁, and Ar₂ include, for example,alkyl groups, alkenyl, alkoxy groups, cyano, aryl, arylalkyl or anyother groups that do not affect the n-type semiconductor properties ofthe film made from such compounds. It is advantageous to avoidsubstituents that tend to interfere with close approach of theconjugated cores of the compounds in a stacked arrangement of themolecules that is conducive to semiconductor properties. Suchsubstituents include highly branched groups, ring structures and groupshaving more than 12 atoms, particularly where such groups or rings wouldbe oriented to pose a significant steric barrier to the close approachof the conjugated cores. In addition, substituent groups should beavoided that substantially lower the solubility and/or volatility of thecompounds such that the desirable fabrication processes are prevented.

Unless otherwise specifically stated, use of the term “substituted” or“substituent” means any group or atom other than hydrogen. Additionally,when the term “group” is used, it means that when a substituent groupcontains a substitutable hydrogen, it is also intended to encompass notonly the substituent's unsubstituted form, but also its form to theextent it can be further substituted (up to the maximum possible number)with any substituent group or groups so long as the substituent does notdestroy properties necessary for semiconductor utility. If desired, thesubstituents may themselves be further substituted one or more timeswith acceptable substituent groups. For example, an alkyl or alkoxygroup can be substituted with one or more fluorine atoms. When amolecule may have two or more substituents, the substituents may bejoined together to form an aliphatic or unsaturated ring such as a fusedring unless otherwise provided.

Examples of any of the alkyl groups mentioned above are methyl, ethyl,propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, hexyl, octyl,2-ethylhexyl, and congeners. Alkyl groups, preferably having 1 to 6carbon atoms, more preferably 1 to 4, are intended to include branchedor linear groups. Alkenyl groups can be vinyl, 1-propenyl, 1-butenyl,2-butenyl, and congeners. Aryl groups can be phenyl, naphthyl, styryl,and congeners. Arylalkyl groups can be benzyl, phenethyl, and congeners.Useful substituents on any of the foregoing or other groups disclosedinclude halogen, and alkoxy, and the like. Preferred substituents areelectron-withdrawing groups such as a cyano group and fluorine groups.

In general Structures I to V (including A and B alternatives), it isadvantageous to have the substituents on Ar₁ and Ar₂ groups that do notinterfere with close approach of the conjugated core of the compounds.It is possible to have multiple substituents but still not interferewith their close approach in accordance with suitable stackinggeometries. It is also possible that properly selected substituents willpromote this desired close approach.

During synthesis both cis and trans isomers are formed because duringthe imidazole ring formation, the amine group can attack either one ofthe two carbonyl groups to form a statistical mixture. The isomers canbe separated depending on the solubility and volatility of the twoforms. If the properties of the two isomers are similar, then separationcan be difficult. For example, GB925409 discloses a process to separatethe cis and trans isomers using solubility difference.

In a preferred embodiment, mixtures of alternate A and B compounds ofeach of Structures I to V are used as the thin film semiconductormaterial. Mixtures of compounds from more than one of Structure I, II,III, IV, and V can also be used, for example, a mixture of compounds ofStructure I-A, I-B, II-A, and II-B.

Specific illustrative examples of useful aryl dicarboxylic aciddiimidazole-based compounds are shown by the formulae below:

Another aspect of the invention relates to a process for making thinfilm semiconductor devices. In one embodiment, a substrate is providedand a layer of the semiconductor material as described above can beapplied to the substrate, electrical contacts being made with the layer.The exact process sequence is determined by the structure of the desiredsemiconductor component. Thus, in the production of an organic fieldeffect transistor, for example, a gate electrode can be first depositedon a flexible substrate, for example an organic polymer film, the gateelectrode can then be insulated with a dielectric and then source anddrain electrodes and a layer of the n-channel semiconductor material canbe applied on top. The structure of such a transistor and hence thesequence of its production can be varied in the customary manner knownto a person skilled in the art. Thus, alternatively, a gate electrodecan be deposited first, followed by a gate dielectric, then the organicsemiconductor can be applied, and finally the contacts for the sourceelectrode and drain electrode deposited on the semiconductor layer. Athird structure could have the source and drain electrodes depositedfirst, then the organic semiconductor, with dielectric and gateelectrode deposited on top.

In yet another embodiment of the present invention, source drain andgate can all be on a common substrate and the gate dielectric canenclose gate electrode such that gate electrode is electricallyinsulated from source electrode and drain electrode, and thesemiconductor layer can be positioned over the source, drain anddielectric.

The skilled artisan will recognize other structures can be constructedand/or intermediate surface modifying layers can be interposed betweenthe above-described components of the thin film transistor. In mostembodiments, a field effect transistor comprises an insulating layer, agate electrode, a semiconductor layer comprising an organic material asdescribed herein, a source electrode, and a drain electrode, wherein theinsulating layer, the gate electrode, the semiconductor layer, thesource electrode, and the drain electrode are in any sequence as long asthe gate electrode and the semiconductor layer both contact theinsulating layer, and the source electrode and the drain electrode bothcontact the semiconductor layer.

A support can be used for supporting the OTFT during manufacturing,testing, and/or use. The skilled artisan will appreciate that a supportselected for commercial embodiments may be different from one selectedfor testing or screening various embodiments. In some embodiments, thesupport does not provide any necessary electrical function for the TFT.This type of support is termed a “non-participating support” in thisdocument. Useful materials can include organic or inorganic materials.For example, the support may comprise inorganic glasses, ceramic foils,polymeric materials, filled polymeric materials, coated metallic foils,acrylics, epoxies, polyamides, polycarbonates, polyimides, polyketones,poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(sometimes referred to as poly(ether ether ketone) or PEEK),polynorbornenes, polyphenyleneoxides, poly(ethylenenaphthalenedicarboxylate) (PEN), poly(ethylene terephthalate) (PET),poly(phenylene sulfide) (PPS), and fiber-reinforced plastics (FRP).

A flexible support is used in some embodiments of the present invention.This allows for roll processing, which may be continuous, providingeconomy of scale and economy of manufacturing over flat and/or rigidsupports. The flexible support chosen preferably is capable of wrappingaround the circumference of a cylinder of less than about 50 cmdiameter, more preferably 25 cm diameter, most preferably 10 cmdiameter, without distorting or breaking, using low force as by unaidedhands. The preferred flexible support may be rolled upon itself.

In some embodiments of the invention, the support is optional. Forexample, in a top construction as in FIG. 2, when the gate electrodeand/or gate dielectric provides sufficient support for the intended useof the resultant TFT, the support is not required. In addition, thesupport may be combined with a temporary support. In such an embodiment,a support may be detachably adhered or mechanically affixed to thesupport, such as when the support is desired for a temporary purpose,e.g., manufacturing, transport, testing, and/or storage. For example, aflexible polymeric support may be adhered to a rigid glass support,which support could be removed.

The gate electrode can be any useful conductive material. A variety ofgate materials known in the art, are also suitable, including metals,degenerately doped semiconductors, conducting polymers, and printablematerials such as carbon ink or silver-epoxy. For example, the gateelectrode may comprise doped silicon, or a metal, such as aluminum,chromium, gold, silver, nickel, palladium, platinum, tantalum, andtitanium. Conductive polymers also can be used, for example polyaniline,poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS). Inaddition, alloys, combinations, and multilayers of these materials maybe useful.

In some embodiments of the invention, the same material can provide thegate electrode function and also provide the support function of thesupport. For example, doped silicon can function as the gate electrodeand support the OTFT.

The gate dielectric is provided on the gate electrode. This gatedielectric electrically insulates the gate electrode from the balance ofthe OTFT device. Thus, the gate dielectric comprises an electricallyinsulating material. The gate dielectric should have a dielectricconstant above about 2, more preferably above about 5. The dielectricconstant of the gate dielectric also can be very high if desired, forexample, 80 to 100 or even higher. Useful materials for the gatedielectric may comprise, for example, an inorganic electricallyinsulating material. The gate dielectric may comprise a polymericmaterial, such as polyvinylidenedifluoride (PVDF), cyanocelluloses,polyimides, etc.

Specific examples of materials useful for the gate dielectric includestrontiates, tantalates, titanates, zirconates, aluminum oxides, siliconoxides, tantalum oxides, titanium oxides, silicon nitrides, bariumtitanate, barium strontium titanate, barium zirconate titanate, zincselenide, and zinc sulfide. In addition, alloys, combinations, andmultilayers of these examples can be used for the gate dielectric. Ofthese materials, aluminum oxides, silicon oxides, and zinc selenide arepreferred. In addition, polymeric materials such as polyimides, andinsulators that exhibit a high dielectric constant. Such insulators arediscussed in U.S. Pat. No. 5,981,970 hereby incorporated by reference.

The gate dielectric can be provided in the OTFT as a separate layer, orformed on the gate such as by oxidizing the gate material to form thegate dielectric. The dielectric layer may comprise two or more layershaving different dielectric constants.

The source electrode and drain electrode are separated from the gateelectrode by the gate dielectric, while the organic semiconductor layercan be over or under the source electrode and drain electrode. Thesource and drain electrodes can be any useful conductive material.Useful materials include most of those materials described above for thegate electrode, for example, aluminum, barium, calcium, chromium, gold,silver, nickel, palladium, platinum, titanium, polyaniline, PEDOT:PSS,other conducting polymers, alloys thereof, combinations thereof, andmultilayers thereof.

The thin film electrodes (e.g., gate electrode, source electrode, anddrain electrode) can be provided by any useful means such as physicalvapor deposition (e.g., thermal evaporation, sputtering) or ink jetprinting. The patterning of these electrodes can be accomplished byknown methods such as shadow masking, additive photolithography,subtractive photolithography, printing, microcontact printing, andpattern coating.

The organic semiconductor layer can be provided over or under the sourceand drain electrodes, as described above in reference to the thin filmtransistor article. The present invention also provides an integratedcircuit comprising a plurality of OTFTs made by the process describedherein.

The n-channel semiconductor material made using the abovearylimidazole-based compounds are capable of being formed on anysuitable substrate which can comprise the support and any intermediatelayers such as a dielectric or insulator material, including those knownin the art.

The entire process of making the thin film transistor or integratedcircuit of the present invention can be carried out below a maximumsupport temperature of about 450° C., preferably below about 250° C.,more preferably below about 150° C., and even more preferably belowabout 150° C., or even at temperatures around room temperature (about25° C. to 70° C.). The temperature selection generally depends on thesupport and processing parameters known in the art, once one is armedwith the knowledge of the present invention contained herein. Thesetemperatures are well below traditional integrated circuit andsemiconductor processing temperatures, which enables the use of any of avariety of relatively inexpensive supports, such as flexible polymericsupports. Thus, the invention enables production of relativelyinexpensive integrated circuits containing organic thin film transistorswith significantly improved performance.

Compounds used in the invention can be readily processed and arethermally stable to such an extent that they can be vaporized. Thecompounds possess significant volatility, so that vapor phasedeposition, where desired, is readily achieved. Such compounds can bedeposited onto substrates by vacuum sublimation or by solventprocessing, including dip coating, drop casting, spin coating, bladecoating.

Deposition by a rapid sublimation method is also possible. One suchmethod is to apply a vacuum of 35 mtorr to a chamber containing asubstrate and a source vessel that holds the compound in powdered form,and heat the vessel over several minutes until the compound sublimesonto the substrate. Generally, the most useful compounds formwell-ordered films, with amorphous films being less useful.

Alternatively, for example, the compounds described above can first bedissolved in a solvent prior to spin-coating or printing for depositionon a substrate.

Devices in which the n-channel semiconductor films of the invention areuseful include especially thin film transistors (TFTs), especiallyorganic field effect thin-film transistors. Also, such films can be usedin various types of devices having organic p-n junctions, such asdescribed on pages 13 to 15 of US 2004/0021204 A1 to Liu et al., whichpatent application publication is hereby incorporated by reference.

Electronic devices in which TFTs and other devices are useful include,for example, more complex circuits, e.g., shift registers, integratedcircuits, logic circuits, smart cards, memory devices, radio-frequencyidentification tags, backplanes for active matrix displays,active-matrix displays (e.g. liquid crystal or OLED), solar cells, ringoscillators, and complementary circuits, such as inverter circuits, forexample, in combination with other transistors made using availablep-type organic semiconductor materials such as pentacene. In an activematrix display, a transistor according to the present invention can beused as part of voltage hold circuitry of a pixel of the display. Indevices containing the TFTs of the present invention, such TFTs areoperatively connected by means known in the art.

The present invention further provides a method of making any of theelectronic devices described above. Thus, the present invention can beembodied in an article that comprises one or more of the TFTs described.

EXAMPLES A. Material Synthesis

The synthesis of arylimidazole-based compounds has been described in GB925,409. In accordance with the invention, a mixture of perylenetetracarboxylic acid dianhydride, such as perylene tetracarboxylic aciddianhydride or naphthalene tetracarboxylic acid dianhydride, which isavailable from Aldrich Chemical Company, 2.5 equivalents ofortho-diamino compound such as 1,2-phenylene diamine (for the Control),4-fluorine-1,2-phenylene diamine for Compounds 13A and 13B (a perylene)or Compounds 37A and 37B (a naphthalene, fluorinated 3,4-diaminopyridine, etc.; zinc acetate in catalytic amounts, and 10-15 ml ofquinoline per gram of dianhydride molecule was heated over 4-5 hours ata temperature of ca. 220° C. The mixture is allowed to cool to roomtemperature, and poured into an excess amount of methanol and theprecipitated solids are collected, filtered and washed with water,methanol and acetone. The solid is then purified by train sublimation at10⁻⁵ to 10⁻⁶ torr. The purified materials contain mixture of cis- andtrans-isomers such as compound 1 (cis) and compound 2 (trans).

The following comparison compounds were prepared and tested in thefollowing examples:

B. Device Preparation

In order to test the electrical characteristics of the various materialsof this invention, field-effect transistors were typically made usingthe top-contact geometry. The substrate used is a heavily doped siliconwafer, which also serves as the gate of the transistor. The gatedielectric is a thermally grown SiO₂ layer with a thickness of 165 nm.It has been previously shown for both p-type and n-type transistors thatelectrical properties can be improved by treating the surface of thegate dielectric. For most of the experiments described here, the oxidesurface was treated with a thin (<10 nm), spin-coated polymer layer, ora self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS).Typically, an untreated oxide sample was included in the experiments forcomparison.

The active layer of arylimidazole-based compound was deposited viavacuum deposition in a thermal evaporator. The deposition rate was 0.1Angstroms per second while the substrate temperature was held at 75° C.for most experiments. The thickness of the active layer was a variablein some experiments, but was typically 40 nm. Silver contacts ofthickness 50 nm were deposited through a shadow mask. The channel widthwas held at 500 microns, while the channel lengths were varied between20 and 80 microns. Some experiments were performed to look at the effectof other contact materials. A few devices were made with abottom-contact geometry, in which the contacts were deposited prior tothe active material.

C. Device Measurement and Analysis

Electrical characterization of the fabricated devices was performed witha Hewlett Packard HP 4145b parameter analyzer. The probe measurementstation was held in a positive N₂ environment for all measurements withthe exception of those purposely testing the stability of the devices inair. The measurements were performed under sulfur lighting unlesssensitivity to white light was being investigated. The devices wereexposed to air prior to testing.

For each experiment performed, between 4 and 10 individual devices weretested on each sample prepared, and the results were averaged. For eachdevice, the drain current (Id) was measured as a function ofsource-drain voltage (Vd) for various values of gate voltage (Vg). Formost devices, Vd was swept from 0 V to 80 V for each of the gatevoltages measured, typically 0 V, 20 V, 40 V, 60 V, and 80 V. In thesemeasurements, the gate current (Ig) was also recorded in to detect anyleakage current through the device. Furthermore, for each device thedrain current was measured as a function of gate voltage for variousvalues of source-drain voltage. For most devices, Vg was swept from 0 Vto 80 V for each of the drain voltages measured, typically 40 V, 60 V,and 80 V.

Parameters extracted from the data include field-effect mobility (μ),threshold voltage (Vth), subthreshold slope (S), and the ratio ofI_(on)/I_(off) for the measured drain current. The field-effect mobilitywas extracted in the saturation region, where Vd>Vg−Vth. In this region,the drain current is given by the equation (see Sze in SemiconductorDevices—Physics and Technology, John Wiley & Sons (1981)):

$I_{d} = {\frac{W}{2L}\mu\;{C_{ox}\left( {V_{g} - V_{th}} \right)}^{2}}$where W and L are the channel width and length, respectively, and COX isthe capacitance of the oxide layer, which is a function of oxidethickness and dielectric constant of the material. Given this equation,the saturation field-effect mobility was extracted from a straight-linefit to the linear portion of the √I_(d) versus Vg curve. The thresholdvoltage, V_(th), is the x-intercept of this straight-line fit.Mobilities can also be extracted from the linear region, whereVd≦Vg−Vth. Here the drain current is given by the equation (see Sze inSemiconductor Devices—Physics and Technology, John Wiley & Sons (1981)):

$I_{d} = {\frac{W}{L}\mu\;{C_{ox}\left\lbrack {{V_{d}\left( {V_{g} - V_{th}} \right)} - \frac{V_{d}^{2}}{2}} \right\rbrack}}$

For these experiments, mobilities in the linear regime were notextracted, since this parameter is very much affected by any injectionproblems at the contacts. Non-linearities in the curves of I_(d) versusV_(d) at low V_(d) indicate that the performance of the device islimited by injection of charge by the contacts. In order to obtainresults that are more independent of contact imperfections of a givendevice, the saturation mobility rather than the linear mobility wasextracted as the characteristic parameter of device performance.

The log of the drain current as a function of gate voltage was plotted.Parameters extracted from the log I_(d) plot include the I_(on)/I_(off)ratio and the sub-threshold slope (S). The I_(on)/I_(off) ratio issimply the ratio of the maximum to minimum drain current, and S is theinverse of the slope of the Id curve in the region over which the draincurrent is increasing (i.e. the device is turning on).

D. Results

Following examples demonstrate that arylimidazole-based compounds can beused as n-channel semiconducting materials for TFT having high mobilityand on/off ratio.

Example 1

A heavily doped silicon wafer with a thermally grown SiO₂ layer with athickness of 165 nm was used as the substrate. The wafer was cleaned for10 minutes in a piranah solution, followed by a 6-minute exposure in aUV/ozone chamber. The cleaned surface was then treated with aself-assembled monolayer of octadecyltrichlorosilane (OTS), made from aheptane solution under a humidity-controlled environment. Water contactangles and layer thicknesses were measured to ensure the quality of thetreated surface. Surfaces with a good quality OTS layer have watercontact angles >90°, and thicknesses determined from ellipsometry in therange of 27 Å to 35 Å.

The purified arylimidazole-based compound, a mixture of Compound 1 and 2as semiconducting material was deposited by vacuum sublimation at apressure of 5×10⁻⁷ Torr and a rate of 0.1 Angstroms per second to athickness of 40 nm as measured by a quartz crystal. During depositionthe substrate was held at a constant temperature of 60° C. The samplewas exposed to air for a short time prior to subsequent deposition of Agsource and drain electrodes through a shadow mask to a thickness of 50nm. The devices made had a 500 micron channel width, with channellengths varying from 20-80 microns.

The devices were exposed to air prior to measurement in a nitrogenatmosphere using a Hewlett-Packard 4145B semiconductor parameteranalyzer. The field effect mobility, FL, was calculated from the slopeof the (I_(D))^(1/2) versus VG plot (left y-axis) to be 6.6×10⁻³ cm²/Vsin the saturation region. The on/off ratio was 8.5×10³ and the thresholdvoltage V_(T)=53 V. Similar saturation mobilities of were measured fromsimilar devices prepared in this way.

Example 2-5

Samples were prepared and tested as in Example 1, except a differentarylimidazole-based material was used as active semiconducting materialfor the TFT. The results are summarized in Table 1.

TABLE 1 Active TFT Material (Compound μ Examples Number) (cm²/Vs) σ (μ)V_(th) (V) σ (V_(th)) I_(on)/I_(off) Comparison 1 C-1A, C-1B 6.6 × 10⁻³2.3 × 10⁻³ 53.28 13.89 8.5 × 10³ 2 13A, 13B 3.4 × 10⁻² 2.7 × 10⁻³ 15.832.29 9.1 × 10⁵ Comparison 3 C-3A, C-3B 1.6 × 10⁻³ 6.4 × 10⁻⁴ 61.90 28.81.7 × 10⁴ 4 37A, 37B 7.6 × 10⁻² 2.1 × 10⁻² 37.46 8.30 2.0 × 10⁶Comparison 5 C-5A, C-5B 3.2 × 10⁻² 1.2 × 10⁻² 53.33 2.67   8 × 10⁵

These examples clearly demonstrate the advantage of inventive Compounds37-A/B and 13-A/B as n-type materials. Accordingly, mobility, Vth andthe on/off ratio are improved over Comparative Examples 3 and 5, clearlydemonstrating the advantageous effect of the fluorine substitution inthe above-described aryl dicarboxylic diimidazole-based compounds ondevice performance.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   20 source electrode-   28 substrate-   30 drain electrode-   44 gate electrode-   56 gate dielectric-   70 semiconductor

1. An article comprising, in a thin film transistor, a thin film oforganic semiconductor material that comprises one or more aryldicarboxylic diimidazole-based compounds selected from the groupconsisting of compounds represented by the following structures:

wherein Ar₁ and Ar₂ are the same or different, and are each carbocyclicor heterocyclic aromatic ring systems, comprising 4 to 60 carbon atoms,fused to each corresponding adjacent imidazole in the structure, whereineach of Ar₁, and Ar₂ are substituted with at least one or more fluorineatoms, and X is an optional organic or inorganic substituent group atany available position on the core nucleus; n is zero or an integer from1 to 8 and m is zero or an integer from 1 to 4, wherein the thin filmtransistor further comprises first and second contact means in spacedapart contact with the thin film and third contact means spaced apartfrom the organic semiconductor material.
 2. The article of claim 1wherein the thin film transistor is a field effect transistor comprisingan insulating layer, wherein the third contact means is a gate electrodeadapted for controlling, by means of a voltage applied to the thirdcontact means a current between the first and second contact meansthrough said layer, the first and second contact means are a sourceelectrode and a drain electrode, and wherein the insulating layer, thegate electrode, the thin film of organic semiconductor material, thesource electrode, and the drain electrode are in any sequence as long asthe gate electrode and the film of organic semiconductor material bothcontact the insulating layer, and the source electrode and the drainelectrode both contact the thin film of the organic semiconductormaterial.
 3. The article of claim 1, wherein the Ar₁ and Ar₂ moietiesare single rings or fused aromatic polycyclic ring systems, includingcarbocyclic, heterocyclic, or hybrid ring systems in which a carbocylicring is fused to a carbocyclic ring.
 4. The article of claim 1, whereinthe organic semiconductor material is at least one compound selectedfrom the group consisting of compounds represented by the followingstructures:


5. The article of claim 4 wherein X is independently cyano or fluorineor any combinations thereof.
 6. The article of claim 1 wherein X isindependently selected from alkyl groups, alkenyl groups, alkoxy groups,halogens, cyano, substituted or unsubstituted aryl and arylalkyl groups.7. The article of claim 1 wherein the Ar₁ and Ar₂ moieties eachindependently comprise one or more further substituents, on one or bothrings, selected from alkyl groups, alkenyl groups, alkoxy groups, cyano,and substituted or unsubstituted aryl and arylalkyl groups.
 8. Thearticle of claim 1 wherein the Ar₁ and Ar₂ moieties each independentlycomprise two or more fluorines substituted on each of the Ar₁ and Ar₂moieties.
 9. The article of claim 1 wherein the Ar₁ and Ar₂ moietieseach independently comprise a benzene, naphthalene, thiophene, quinolineor pyridine ring system each substituted with one or more fluorines. 10.The article of claim 1 wherein the organic semiconducting material iscapable of exhibiting electron mobility greater than 0.05 cm²/Vs. 11.The article of claim 1, wherein the thin film transistor has an on/offratio of a source/drain current of at least 10³.
 12. The article ofclaim 1, wherein said first, second and third contact means comprise,respectively, source, drain, and gate electrodes, each independentlycomprising a material selected from doped silicon, metal, and aconducting polymer.
 13. An electronic device selected from the groupconsisting of integrated circuits, active-matrix display, and solarcells comprising a multiplicity of thin film transistors according toclaim
 1. 14. The electronic device of claim 13 wherein the multiplicityof the thin film transistors is on a non-participating support that isflexible.